1. Field of the Invention
The present invention relates to a power supply device for an image forming apparatus that uses an electrophotographic process, and an image forming apparatus.
2. Description of the Related Art
A power supply device is already known that applies an output voltage to a transfer member that transfers a toner image formed on an image bearing member onto a recording material. For example, in an image forming apparatus according to an electrophotographic method in which a transfer roller comprising roller-shaped conductive rubber wound around a metal shaft is used as a transfer member, the transfer member is rotationally driven in conformity with the process speed of a photosensitive member. A direct current bias voltage is used as the voltage applied to the transfer member, and in order to perform favorable transfer using the transfer roller, normally a current of about 10 μA is applied.
To generate the high voltage required for image formation as described above, conventionally a wire-wound type electromagnetic transformer has been used. However, the electromagnetic transformer comprises a copper wire, a bobbin, and a magnetic core, and when used in an image forming apparatus of the above kind of specifications, since the output current value is a minute current of about 10 μA, it has been necessary to reduce leakage current at each part to a minimum. It has thus been necessary to insulate the winding of the transformer with a mould or the like, and since a large transformer has been required relative to the power supply, this has proved a hindrance to miniaturization and lightening of high voltage power supply devices (power supply devices for outputting a high voltage).
Therefore, to compensate for these shortcomings, a unit that generates a high voltage using a high-output piezoelectric transformer that is thin and lightweight is being studied (Japanese Patent Laid-Open No. 11-206113). That is, by using a piezoelectric transformer that is made from a ceramic material, it is possible to generate a high voltage with efficiency that is greater than that of an electromagnetic transformer. Furthermore, since it is also possible to provide a distance between electrodes on a primary side and a secondary side, it is not necessary to perform a special mold process for insulation, and the excellent characteristics gained from a high-voltage generator being made small and lightweight can be obtained.
The characteristics of a piezoelectric transformer are generally that an output voltage forms a shape with a wide base that reaches a maximum at a resonance frequency f0 as shown in FIG. 9, and control of the output voltage by frequency is possible. When controlling the output voltage at a driving frequency that is higher than the resonance frequency f0, the driving frequency is changed from a high frequency to a low frequency to increase the output voltage of the piezoelectric transformer. Conversely, when controlling the output voltage at a driving frequency that is lower than the resonance frequency f0, the driving frequency is changed from a low frequency to a high frequency to increase the output voltage.
In a circuit using the piezoelectric transformer as described in Patent Document 1, the operating frequency range of a voltage controlled oscillator (VCO) is set to a range that includes the resonance frequency f0. More specifically, control is performed in a voltage range including a voltage larger than the voltage of an unwanted resonance frequency (resonance frequency other than f0; hereunder referred to as “spurious frequency”) that is generated due to the structural characteristics of the piezoelectric transformer and smaller than a voltage at the resonance frequency f0. It is known that inputting a voltage that does not include harmonic contents such as a sine wave into the piezoelectric transformer is effective for suppressing occurrence of this spurious frequency.
However, according to the aforementioned prior art, the piezoelectric transformer has a configuration that is liable to be affected by a spurious frequency because the voltage waveform includes harmonic contents as shown in FIG. 8. In a case in which it is necessary that the required voltage range is wide (for example, when using a transfer roller with large load fluctuations caused by the environment), control has been difficult at the low voltages at which the transformer is liable to be affected by a spurious frequency. There is also a problem that a method that drives a piezoelectric transformer with a sine wave in order to suppress the occurrence of a spurious frequency involves high costs.